MAVLink passthrough in language wrappers

[julianoes]

It would - of course - be nice to have MAVLink passthrough available in language wrappers.
However, that's not straightforward because it would mean we have to:

1. Either generate .proto files for all MAVLink messages and then generate the API for all of them. This would be nicely type-safe in these languages but probably gets quite bloaty.
2. Or expose some sort of interface that packs the MAVLink messages with runtime checks instead of compile team. We could do that with some sort of interface that allows you to set a message and all the fields using string setters/getters. Something along these lines:

service Passthrough Service {
    rpc SendMessage(SendMessageRequest) returns (SendMessageResponse)
    rpc SubscribeReceiveMessage(SubscribeReceiveMessageRequest) returns (stream ReceiveMessageResponse)
}

message SendMessageRequest {
    String msgid = 1;
    repeated Field fields = 2;
}
message SendMessageResponse {
    Result result = 1
}

message SubscribeReceiveMessageRequest {
    String msgid = 1;
}
message ReceiveMessageResponse {
    String msgid = 1;
    repeated Field fields = 2;
}

message Field {
    String fieldid = 1; // _x for arrays
    int64 int_data = 2;
    double float_data = 3;
    String string_data = 4;
}

message Result {
    enum PasshtroughResult {
        UNKNOWN
        SUCCESS
        WRONG_DATA_SET
        DATA_OBERFLOWN
        CONNECTION_ERROR
}

Note, we might also be able to use the oneof proto thing.